1. Field of the Invention
The present invention is generally in the field of electrical circuits. More particularly, the invention is in the field of bias circuits.
2. Background Art
Microphones, such as microphones used in cell phones, computers, and handheld audio devices, typically require a stable bias voltage to operate. For proper microphone operation, the bias voltage needs to remain at a constant level that is independent of the load current, i.e. the amount of current that flows into the microphone. Furthermore, the bias voltage needs to have low noise to ensure sufficient audio quality and should be very insensitive to any variation on the power supply to maintain a stable voltage across the microphone.
A conventional bias circuit for providing a microphone bias voltage can include a high impedance operational amplifier coupled to a transistor, such as a PMOS transistor. The conventional bias circuit can also include a feedback loop where the output of the bias circuit is sensed using a resistor network and compared to a stable, low noise reference voltage. The operational amplifier can force the feedback voltage to be equal to the reference voltage by appropriately adjusting the gate voltage of the transistor. However, in the conventional bias circuit, noise on the transistor's supply voltage (e.g. VDD) can easily be coupled to the output of the bias circuit, thereby undesirably affecting microphone performance. Thus, the conventional bias circuit typically exhibits poor power supply rejection (PSR), which refers to the capability of the bias circuit to reject variations in supply voltage caused by noise.